Echinoderms

Animals with spiny skins and horrible habits!

EchinodermsThe echinoderms are the ‘stars’ of the marine world. They include starfish, sea urchins and crinoids or sea lilies. Many are found as intricately-patterned fossils, dating back over 500 million years.  The family name comes from the Greek words ‘echino’ meaning spiny, and ‘derm’ meaning skin.


 

Five-star creature

Bloody Henry starfishEchinoderms are usually made of five sections (or odd numbers of loges) arranged around a central disk. This is known as five-fold symmetry. Most starfish have five arms, sea urchins have five sides, and a crinoid or sea lily is basically a starfish on a stalk! The sea cucumber doesn’t look like the other echinoderms, but it too has five-fold symmetry, with five double rows of tube feet running along its body. Fossils of sea cucumbers are rarely found.


 

Feet that suck

Seven Armed StarfishAnother distinctive feature of this group is that they have tubed feet. You can clearly see these in starfish and sea urchins when they are in the water. The feet run in rows along the arms or around the shell. Each foot has a sucker on the end that attaches to a surface. By extending and retracting its feet, the animal drags itself along.

But the feet aren’t just used for walking – read on to find out more…


 

Starfish and their horrible habits!

Tubed feetStarfish might look beautiful, but they have disgusting eating habits. They use their tubed feet to prise apart shells such as clams. Then the stomach of the starfish pops out of its mouth and into the shell. There it digests the softs parts of the clam, before sliding back into its own body. Yuk!

But it gets worse. Starfish don’t have an anus – so they poo by spitting the waste material out of their mouths! They also secrete waste products through their tubed feet.


 

Beautiful brittle stars

Brittle stars are delicate starfish with long, slender arms. They live on the sea floor, often in huge numbers. They can feed by holding their arms up to catch food in the passing currents. You can see fossilised brittle stars in our collections. In some, the arms are pointing the same way, suggesting that they were aligned by currents on the sea floor.

Brittlestars

Super-spiny sea urchins

UrchinSea urchins have taken the spiny skin to extremes as anyone who has stepped on one will know! The spines are there for protection, and can contain poison. Some urchins have even developed their spines into clubs, each armed with many more spines.  Urchins can be round in shape or irregular like the sea potato. This unusual creature burrows in the sea bed, then uses its furry spines and tubed feet to make a snorkel to suck in sea water and sediment. It extracts the nutrients, then excretes the waste.

The sea urchin’s spines fall off after death, so most fossils consist of the test (shell) or scattered spines. The delicate test is often crushed, but some in our collections are beautifully preserved.

Fascinating fact!

To make a sea urchin, take a starfish and fold the arms into a point. You now have the shell or ‘test of an urchin.

Urchin

In the photograph above, you can see the how the five arms come together to form a shell or test of the urchin, but look carefully and you will also see the tubed feet protruding in rows from the darker areas. The point where the five arms meet is the anus!

Crinoids or ‘sea lilies’

Crinoids are marine animals – the name sea lily comes from their resemblance to a plant. They live attached to the sea bed or on floating drift wood. Crinoids have been around for almost 500 million years and still exist today. They’re usually found in tropical waters, but also around the UK, especially the west coast of Scotland.

Featherstar

Some crinoids have a long stalk, which extends down from the body (calyx) to a claw-like anchor which attaches the animal to the sea bed, rocks or wood. Others have hardly any stalk and are known as feather stars, see image above. The arms of crinoids branch into a fine filter that catches plankton in the water. Tubed feet run along the inside the arms which carry food to the mouth (on top of the calyx).

Usually it’s the stalk of the crinoid that’s found as a fossil, but some in our collection also show the arms. Often many crinoids are packed into one rock.

Marine images Richard Edmonds, shot around Holm, Stornoway.